During the War Spear ceased teaching at the RCA (which was evacuated to Ambleside in 1940) and served for three years as a fire-fighter in Shepherds Bush. This Wartime period was not a completely redundant time for Spear from a professional point of view - he assisted on the removal, for protection, of windows at Canterbury Cathedral.

He also experimented with new designs which were more modern in feel, a change that he hoped would 'give the feeling of the subject with the greatest simplicity and with the elimination of all details.' and devised his distinctive monogram of an interlocking S with a sideways F.

When the War ended Spear gained so many commissions - to replace stained glass windows destroyed during the Blitz.- that by 1947 he was employing four assistants.

Francis Spear is an important figure in twentieth century English stained glass. His working career covers 50 years, from 1922 when he began working with Martin Travers, to 1972, when he ceased teaching at Reigate School of Art.

During his career, he designed windows for over 130 locations; and a short list of notable designs include his earliest window, at Warwick School (1925), St. Olave's in the City (1929), Snaith (1936), Beckenham (1948), Canterbury (1949), Glasgow Cathedral (1951, 1953, 1958), Highbury (1955), Westgate (1960) and Penarth (1962).

The collection of the Prints and Drawings department of the Victoria & Albert Museum own all of the surviving cartoons for the 300 extant windows he produced over his fifty year long career.